The present invention relates to gas chromatography, particularly to portable gas chromatograph systems and more particularly to a multiple parallel hand held gas chromatograph (GC) system wherein each independent GC unit within the system has its own injector, separation column, detector and oven retained in a light weight compartment.
Gas chromatography (GC) has proven to be a reliable method for identifying unknown chemical mixtures. In a GC, an unknown chemical mixture is first injected and carried by a carrier gas into its separation column. In the separation column, the chemical mixture is distributed between two phases, one a mobile phase and the other the stationary phase. The mobile phase is transported by a carrier gas in the separation column, and the stationary phase is adsorbed into a solid from the flowing mobile phase. Based upon the kinetics of the adsorption-desorption process in the gas-solid interface in the column, different chemicals achieve different moving speeds in the carrier gas. Within a precise column length, the chemical mixture can be separated into various components and recorded by the detector. For the conventional GC, detector sensitivities are at about parts per million for thermal sensitivity detectors and parts per billion for various ion cell detectors. Chemical components are identified through their retention times--the traverse time for each of them as they pass through the separation column.
The separation efficiency of the GC column is related to the degree to which a solute band broadens (which is a function of the width of the peak, w) relative to the length of time the band requires to traverse the column (its retention time t.sub.R). The number of theoretical plates N is defined as, EQU N=16(t.sub.R /W).sup.2 (1)
Or, the number of effective theoretical plates is defined as, EQU N=5.56[(t.sub.R -t.sub.M)/w.sub.1/2 ] (2)
where t.sub.M is the duration for nonabsorbed gases to traverse the separation column and w.sub.1/2+L the half linewidth of a signal peak. The efficiency of a separation column is measured by the number of theoretical plates per unit length or the "height equivalent to a theoretical plate" h (HETP), EQU h=L/n (3)
Or, the "height equivalent to one effective theoretical plate" H (HEETP), EQU H=L/N (4)
In the conventional GC, the separation column is usually formed by a glass capillary with a diameter ranging from 150 to 300 microns and a length of around 100 meters. The values of h or H are on the order of millimeters. To achieve this high column efficiency, the temperature of the separation column needs to be uniform. It must be placed in a large, well-insulated oven. Because of this, conventional GCs are bulky, have high power consumption, and are slow in response.
Recently, efforts have been directed to developing portable GCs for field operations, even miniaturized to a hand-held size.
Performance requirements on field portable GCs are more stringent than those for laboratory GCs. Not only do instruments need to be smaller, more rugged, and consume low power, they also need to be a higher speed for analysis. In the conventional GC, the determination of the separated gas is by its retention time. Some gases may have quite similar retention times in a particular coated separation column, but not in other coated separation columns. Thus, there are normally two or more exchangeable separation columns in each conventional GC. These exchangeable separation columns share a common injector, oven, and monitoring system. Thus, it takes a considerable period of time to change from one separation column to another especially if the temperature of the oven is quite different.
The hand-held GC has a wide appeal for numerous applications, such as toxic gas monitors, pollution detectors, reaction gas analysis, and law enforcement usage. To miniaturize a conventional GC, one first must first miniaturize its separation column and oven assembly.
The separation column and oven assembly can be greatly simplified when make from a highly thermal conductive material. Silicon has higher thermal conductivity than glass, and its surface under ambient conditions will naturally form silicon dioxide, an inert material which is ideal for forming the inner liner of the separation column. To form a highly efficient separation column on a silicon wafer, the column must have (a) a circular cross section, (b) an extremely smooth inner surface, and (c) proper diameter in terms of its length. The heater can then be made directly on the same silicon wafers or on a separate one. Such a hand-held GC is described in UCRL-JC-130439 "A High Performance Hand-Held Gas Chromatograph," Conrad M. Yu, November 1998.
Because of the small sample size, the thermal conductivity detector needs to be extremely sensitive. The heat capacity of the heater and the total amount of conductive heat loss other than that due to the carrier gas has to be minimized. This device is small so it can allow for the direct interconnecting of the thermal detector into the gas stream. All this is done by MEMS (Micro-Electro-Mechanical-System) technology.
Fabrication of a miniaturized separation column using MEMS wafer alignment and bonding, column coating and conditioning, fabrication of a miniaturized thermal conductivity detector using MEMS, assembly of the main components, the sample injector, the capillary column, and the detector and related electronics, as well as test results, for a miniature silicon GC is described in above-referenced UCRL-JC-130439; and such is incorporated herein by reference thereto.
The present invention involves a hand-held multiple parallel gas chromatograph (GC) system which includes several independent GC units each having its own injector, separation column, detector, and oven, mounted in a light weight housing. Each GC unit operates independently and simultaneously. Different retention times for the same gas may be measured because of different coatings in the different separation columns. The multiple parallel GC system of the present invention is constructed utilizing MEMS technology, which enables miniaturization of various components of the system.